The Interior Department's tentative list of "undeveloped coastal barriers" keeps growing. The department recently added 55 sites to the January list of 159 fragile, sandy coastal areas that probably are not suitable for development.

Tracts that ultimately get that designation will not be able to qualify for federal flood insurance after Oct. 1, 1983, a prohibition stipulated by Congress in the 1981 budget reconciliation act. In addition, Congress is considering a ban on federal subsidies for sewers, highways, bridges and other activities that encourage development. The Senate Environment and Public Works Committee recently passed such a measure, and a House subcommittee has scheduled hearings on a similar one later this month.

Such moves are strongly opposed by the real estate industry, which could lose millions, if not billions, of dollars. But the issue is one of the few on which the cost-conscious Reagan administration and environmentalists have joined forces.

Interior said that since it released its first list in January it has received recommendations that nearly 200 additional sites be included. The 55 latest additions include 19 tracts in Florida, 11 in Massachusetts, five in Connecticut and four in New York. In this area, the department increased the designated area on Virginia's Assawoman Island. An accompanying draft environmental impact statement found that the proposed action on the 214 sites would have a positive effect on the marine life and the shore area and could save the federal government a total of $5 billion over the next 20 years. But the draft statement estimated that in a "worst case scenario" it could cost developers $650 billion over 20 years.

Interior plans to have the revised maps of all 214 sites ready for state inspection by July 1. In August it is to submit its proposed list to Congress, and after a 60-day comment period a final list will be released.